This invention relates to a guide assembly for guiding the casting belts of a twin-belt continuous casting mold. The casting belts bound the mold chamber at the top and bottom, while the mold chamber is laterally bounded by segmented side dams which adjoin the casting belts. In the zone of the mold entrance and mold outlet, there are provided respective upstream and downstream end drums for positioning and supporting the casting belts and between the upstream and the downstream end drums there is arranged a series of support rollers which engage the respective casting belt opposite the mold chamber. Between the end drums at the mold inlet (the upstream end drums) and the first support roller downstream thereof, as viewed in the casting direction, with each casting belt there is associated a ribbed additional support which engages the respective casting belt transversely to the casting direction in the zone of the mold chamber and the side dams.
In the casting of non-ferrous metals--particularly copper, aluminum and zinc--the molten metal is introduced into the mold chamber by means of an open, trough-like tundish whereby in the inlet (entrance) zone of the mold chamber only the lower casting belt is stressed, and such stresses are derived mainly from the weight of the molten metal. Only in the mid zone or the outlet zone of the mold chamber which is inclined slightly downwardly from the horizontal, is a ferrostatic pressure present which exerts pressure forces, with the intermediary of the casting skin, on all mold walls and thus also on the segmented side dams.
If, for the purpose of achieving a high-quality product, molten steel is introduced into the mold chamber from an intermediate vessel through a tubular casting nozzle with the exclusion of air, all mold walls are exposed to a ferrostatic pressure already in the inlet zone of the mold chamber. Such ferrostatic pressure may reach 0.75 bar, dependent upon the parameters of the molten column.
Twin-belt continuous casting molds are usually equipped with an upper frame and a lower frame carrying the upper and lower casting belts, respectively. The frames are supported on one another by means of spacer pins. A clearance in the order of magnitude of up to 0.1 mm set by this arrangement between the lower edge of the support rollers of the upper casting belt and the upper casting belt is needed to allow compensation for lateral displacements of the two casting belts by a lateral edge guidance and for shrinkages within the mold chamber by an inclined positioning of the guide arms for the side dams. Disadvantageously, in these prior art constructions the molten steel under pressure penetrates in the inlet zone of the mold chamber into the clearance between the upper casting belt and the side dams, solidified in the clearance to form thin, torque-like deposits and thereby increases the height of the side dams. Upon passage of the subsequent support rollers the upper frame of the twin-belt continuous casting mold is lifted whereupon, as a result, the sealing gap set between the casting nozzle and the mold chamber increases, allowing the molten steel to escape which necessitates the interruption of the casting operation.
The formation of the tongue-like deposits--whose initial thickness of 0.3 to 0.5 mm is several times greater than the height of the clearance set by the above-noted spacer pins--is, pursuant to the recognition on which the invention is based, caused preponderantly by the temperature distribution prevailing within the casting belts. As a result of the cooling of the casting belts, their lateral zones, externally of the mold chamber, remain cold during the casting process, while the central zones in the region of the mold chamber have an average temperature which is at least 100.degree. C. higher. Further, the casting belt temperature prevailing in the zone of the mold chamber increases by more than 100.degree. C. in the direction of belt travel. This non-uniform temperature distribution in the transverse and longitudinal directions of the casting belts and the resulting different length variations (particularly if the distance between the upstream and the downstream end drums is more than 4 m) lead, in the absence of particular measures, to uncontrollable casting belt deformations in the zone of the casting chamber (that is, in the zone between the side dams) which allow a penetration of the molten steel between the casting belt and the side dams.
An appearance of unsealed locations between the mold walls is in some instances also caused by the fact that the components cooperating in the zone of the mold chamber (such as casting belts, side dams, support rollers and support roller bearings) necessarily have to be manufactured and installed with tolerances. The thus-resulting deviations add up in disadvantageous cases such that in the longitudinal direction of the mold chamber between the casting belts and the side dams there may appear gaps in the order of magnitude of several tenths of a millimeter in a non-uniform distribution.
German Auslegeschrift (Published Examined Application) No. 1,433,036 discloses a guide assembly of the above-outlined type which, at the mold chamber inlet, in the zone behind the end drums is equipped with additional supports constituted by additional ribbed support rollers which support the upper and the lower casting belts while projecting in part into the upstream end drums. The additional support rollers as well as the other support rollers, however, are immovably supported at the associated upper or lower frame of the twin-belt continuous casting mold and therefore they are incapable of preventing the above-noted casting belt deformations and the appearance of gaps as well as the disadvantages resulting therefrom.
German Auslegeschrift No. 1,558,259 and German Offenlegungsschrift (Published Non-examined Application) No. 1,758,957 propose to support the endless casting belts of continuous casting molds between the end drums by means of resiliently positioned support rollers. The disclosed embodiments, however, have a vertically oriented mold chamber with four bounding casting belts and are equipped with a tubular mold immediately succeeding the casting belts resulting in the formation of a load supporting casting skin. The prior art is not concerned with arrangements to improve the guide assembly at the casting belts of a twin-belt continuous casting mold equipped with side dams with a view towards operational safety.